1. Field of the Invention
The invention relates to a display device, and particularly to a display device which conducts an instrument indication, for example, a speed meter or a tachometer, a warning indication, a facility indication, and the like in transportation such as an automobile, an airplane, and a ship.
2. Description of the Related Art
Among display devices of such a kind, for example, an instrument panel of an automobile or the like is configured by a combination of mechanical meters having a pointer, and warning lamps using an electric bulb.
Such a mechanical meter is complicated in structure and has a number of parts. The production of a mechanical meter requires much labors. Furthermore, the complicated structure causes much troubles.
Electrical specifications of a mechanical meter require a large number of wires. Since there is not a space sufficient for laying conventional electric wires of such a large number in bundle, an expensive flat wiring member such as a PCB (Printed Circuit Board) or an FPC (Flexible Printed Circuit board) is usually used.
As another display device, a two-sided display unit which belongs to the field of a so-called flat display and which can conduct display on both the front and rear faces is used. Conventionally, as described in Japanese Patent Unexamined Publication No. SHO. 59-121744, a two-sided display unit of this kind is configured by a fluorescent display tube having a structure in which an anode is made nontransparent or display light is blocked by a nontransparent filter, thereby selectively displaying display patterns of the front and rear face sides.
In another display unit, as described in Japanese Utility Model Unexamined Publication No. HEI. 4-16861, a fluorescent display tube has a structure in which display light is blocked by a nontransparent mask member, thereby selectively displaying display patterns of the front and rear face sides.
However, each of the prior art two-sided display units disclosed in the publications has a structure in which a self-luminous element consisting of a fluorescent display tube is used, and hence has a defect that the two-sided display unit itself is expensive.
In such a display device for a vehicle, recently, attempts have been made to form a virtual image of a display image of a display unit under magnification through an optical element such as a lens, in place of viewing a real image displayed on the display unit, thereby improving the visibility and hence enhancing the value as a product.
An example of a system which conducts a display in such a manner and is widely used is the HUD (Head Up Display) system. In an HUD used in an automobile, an image of a display element irradiated by a light source is projected onto a combiner which is disposed on the side of the driver so as to face a lower region of a windshield glass, or on the windshield glass itself, and a display is conducted by using the combiner or the windshield glass (hereinafter, represented by the combiner) as a screen. When display contents such as shown in FIG. 23 of an instrument board disposed in a front-and-lower portion of the driver's seat are displayed on a projection face of the combiner by the HUD, the driver can see the display contents of the instrument board with looking ahead, without downward directing the line of sight while driving the automobile, because the combiner is disposed on the upper face of an instrument panel. From the view point of traffic safety, this display is very advantageous particularly for a person such as an old person or a beginner who is entirely devoted to observation of the road state during driving.
An HUD is used as an auxiliary display device for the instrument board, a navigation system, or the like. In order to conduct a display by means of the HUD, a display device dedicated to the HUD must be prepared. Consequently, an automobile equipped with an HUD in addition to an instrument board is very expensive.
In order to reduce the cost, it may be contemplated to employ a configuration in which an instrument board is not disposed and display contents of an instrument board are incorporated into an HUD so that a display is conducted by using the HUD only. However, this configuration has the following drawback. In the case where a wide field of view is to be obtained in a windshield glass, a combiner obstructs the field of view. When a projection face 15 of a combiner 14 is laid down so as to turn down the display as shown in FIG. 24, the driver cannot get information produced by the instrument board and hence an accident may be caused.
Although a parking brake is pulled and the HUD conducts a warning indication on the combiner 14, for example, the display may be turned down in order to obtain a wide field of view as described above. In this case, the driver continues to drive the automobile without being aware of the warning contents relating to the parking brake.
Therefore, an instrument board which always conducts a display, or an indicator equivalent to such an instrument board must be disposed. Among the display contents of such an instrument board, particularly, warning indications such as the above-mentioned parking brake warning, charge warning, ABS warning, and engine control unit warning are preferably performed also on the projection face 15 of the combiner 14 in view of safety of an old person or a beginner.
As described above, however, an automobile equipped with both an instrument board and a display device dedicated to an HUD is very expensive.
Further, when plural display information sets are collectively displayed in one region, recognizability is impaired. In the case where a display of a digital clock and a display of the travel distance which is similarly in the digital form are juxtaposed, for example, the displays may be confusedly erroneously recognized. Such a configuration may be sometimes inappropriate also from the viewpoint of the design of an automobile.
Moreover, the opening of the instrument panel itself is reflected on the windshield glass, and the display unit inside the instrument panel is reflected on the windshield glass through the opening. There arises a problem in that such reflected images obstruct the view of the driver.